These presses for injection moulding usually comprise a base and four horizontal columns arranged above the base, in conformity with the vertices of a quadrilateral, and which extend between a closing or reaction plate that can be positioned on the base and resting against reaction blocks applied at one end of the columns and a stationary mould-holder plate, fixed onto the base and resting against stop blocks at the other end of the columns. A mould-holder plate is conducted along the columns, guided on the base and displaceable towards and away from the fixed mould-holder plate.
In such presses, the locking/releasing movements to close the mobile mould-holder plate against the fixed mould-holder plate are obtained by means of a control unit, known as a double-toggle unit, composed of hinged rods or connecting rods extending and withdrawing when operated by at least one hydraulic cylinder.
On conventional presses, the toggles normally fold up towards the axis of the press, that is towards the inside of the columns. The rods or connecting rods of the toggles are connected on one side to the mobile mould-holder plate and on the other side to a mobile plate or to a so-called crosshead plate, operated by at least two power cylinders and conducted along an additional column between the horizontal guiding columns of the machine. However, the presence of a crosshead plate and/or additional columns between the main columns and especially the folding back of the toggles towards the axis of the machine, as well as obstructing access inside the structure for maintenance or otherwise, also limit the opening stroke of the mobile mould-holder mould. This limitation means that a press can be used only for bodies with a maximum depth in compatibility with the travel of the mobile mould-holder plate even though it could potentially be suitable for moulding even deeper bodies. In other words, for moulding deeper bodies it is then necessary to resort to larger, bulkier and more powerful presses, which are also more costly and which can have an adequate stroke for the requirements, even though in theory a smaller press, less bulkier and with a lower cost, being able to have a greater stroke, would be able to mould bodies as deep as the larger press.